As urban centers increase in size, and traffic congestion becomes more common, the need for accurate and up-to-date traffic information also increases. Traffic surveillance relies primarily on traffic sensors, such as inductive loop traffic sensors that are installed under the pavement. Alternatively, video sensors may also be used to obtain traffic information.
Residing underground, inductive loop sensors are expensive to install, replace and repair because of the associated roadwork required. Moreover, such roadwork also causes traffic disruptions. Video sensors, on the other hand, are cheaper, but have other drawbacks, such as an inability to operate in the dark or in weather that impairs visibility, such as fog or snow.
To overcome these drawbacks, radar sensors have been employed to obtain traffic information. Radar sensors typically transmit low-power microwave signals at the traffic, and detect vehicles based on the reflected signals. Radar sensors are generally cheaper than inductive loop traffic sensors, and, unlike video sensors, operate in the dark and in a wide range of weather conditions.
For proper and stable operation, frequency modulated continuous wave (FMCW) radars must transmit a stable and preferably linear frequency sweep. Any non-linearities of the sweep can reduce range resolution. Further, changes in the df/dt sweep slope due to temperature drift may reduce the accuracy of ranges measured, and shifts in the center frequency due to temperature changes can push the transmission signal out of the FCC allocated band.
One solution to this problem involves digitally synthesizing the radar transmit signal such that at all times it is derived to be some numerical multiple of a fixed low crystal-controlled reference frequency; however, this approach tends to be expensive both in terms of production cost and power consumption.
Another approach adopted is to rely on an analog oscillator to generate the transmit signal. In this case the frequency modulation would be performed by a single varactor diode. The main drawbacks of this approach are non-linearity and temperature drifts. One solution is to provide a frequency generation oscillator (FGO) circuit with a memory chip. This memory would be loaded with numbers defining a function derived from the polynomial N(x)=Ax2+Bx+C in which the A coefficient denotes the non-linear part, the B coefficient denotes the primary modulation slope and the C coefficient relates to the center frequency.
By testing each transceiver after manufacture, its A, B and C coefficients can be determined and recorded in the memory resulting in a linearized sweep, and substantially correct slope and center frequency as required. Further, through a combination of real-time temperature measurement and statistical analysis of batches of microwave transceivers, temperature correction can be applied to the A, B and C coefficients so as to stabilize the sweep function and center frequency by compensating for variation in these coefficients due to temperature. However, despite this compensation, there may be substantial drift of the df/dt slope. Accordingly, this approach while being cheaper both in terms of cost and power consumption than the above-described digital solution, may suffer from the drawback of reduced accuracy.